This invention relates to non-iron base alloy compositions and methods and, in a preferred embodiment, to nickel and cobalt base alloys. In a more preferred aspect, this invention relates to compositions and methods employing dopants in nickel and cobalt base alloys as a means of modifying and improving the elevated temperature oxidation resistance of the resulting alloy compositions.
Commerical nickel and cobalt base high temperature alloys resist oxidation attack by forming a protective oxide surface scale during elevated temperature exposure to atmospheres containing oxygen. The protective scale limits the amount of oxygen, as anions, available for reaction with the host alloy. Although this protection is substantial, it is not total and oxidation failure typically occurs by internal oxidation. In one failure mode, grain boundary oxidation leads to rapid deterioration of mechanical properties. A second failure mode involves progressive formation of new oxide at the outer scale surface or the inner scale surface at the interface between the host alloy and scale leading to thick scale growth and eventual spallation. Failure mode in this case is attributed to reduced section thickness as the host alloy is consumed. Often, both of these failure modes operate at the same time.
This invention deals with a cost effective method of improving the protective capacity of oxide scales formed on a broad range of non-iron base alloys such as wrought or cast nickel or cobalt base heat resistant alloys; the present invention also relates to methods of preparing such alloys.
By way of summary, the compositions and methods of the present invention relate to the discovery that certain elements can be added to non-iron base alloy materials to dramatically improve their resistance to oxidation. More particularly, the invention relates to the discovery that the addition of these elements (referred to herein as "dopants") yield lower cost materials suitable for use in heretofore impractical environments.
Accordingly, the compositions and methods of the present invention relate to non-iron base alloy compositions exhibiting improved resistance to oxidation which employ:
(a) a first non-iron metal alloy element; PA1 (b) at least one second non-iron alloy element selected from the group consisting of nickel, chromium, molybdenum, manganese, silicon, carbon, vanadium, cobalt, copper, nitrogen, titanium, zirconium, aluminum and mixtures thereof; wherein said first metal alloy element is present at a weight percent level greater than said second non-iron alloy element; and PA1 (c) an effective amount of a dopant selected from the group consisting of lithium, sodium, potassium, yttrium, lanthanum, cerium, calcium, magnesium, barium, aluminum, beryllium, strontium and mixtures thereof.
In a preferred embodiment, the compositions and methods of the present invention employ barium, calcium, lithium, lanthanum/cerium, magnesium potassium and sodium or mixtures thereof are added to the alloy as dopants.
The compositions and methods disclosed herein involve the addition of small quantities of elements appearing, for the most part, in Groups IA, IIA and IIIB of the Periodic Table to the base alloy composition. These elements, as ions, enter into the protective oxide scale and modify predominantly anion and to a lesser extent cation transport through the oxide scale, greatly reducing the amount of oxidation observed due to elevated temperature exposure.